Photodetectors are widely used for light-sensing and imaging applications. For sensing of low light levels, the sensitivity of photodetectors is limited by various sources of noise which result in a dark current in the absence of any light incident onto the photodetectors. To reduce the dark current and thereby improve the sensitivity of a photodetector cooling of the photodetector is generally required.
The photodetectors of the present invention overcome the limitations of the prior art by providing a barrier layer which separates a light-absorbing layer which is doped n-type or p-type and a contact layer having a doping type which is opposite that of the light-absorbing layer. The barrier layer blocks a flow of minority carriers from the contact layer to the light-absorbing layer and also eliminates surface leakage currents, thereby reducing the dark current in the photodetectors of the present invention.
The photodetectors of the present invention also utilize a graded layer between the light-absorbing layer and the barrier layer so that a valence band energy level of the barrier layer need not be substantially equal to the valence band energy level of the light-absorbing layer when the light-absorbing layer is n-type doped, and so that a conduction band energy level of the barrier layer need not be substantially equal to the conduction band energy level of the light-absorbing layer when the light-absorbing layer is p-type doped. The use of a graded layer in the photodetectors of the present invention relaxes the requirements on the barrier layer and permits the use of the III-V compound semiconductor materials indium aluminum gallium arsenide (InAlGaAs) and indium phosphide (InP) for the barrier layer when the light-absorbing layer comprises indium gallium arsenide (InGaAs).
These and other advantages of the present invention will become evident to those skilled in the art.